An electromagnetic rapid start fluorescent lamp typically utilizes continuously excited filaments to provide a thermionic emission of electrons that aid in lamp starting, the excitation heating the filaments. Such a lamp uses a ballast which applies an output voltage across the lamp which will "strike" (i.e., initiate light emission from) the lamp when the filaments are heated, but which will not strike the lamp when the filaments are not heated. If a higher voltage ballast is used, the lamp can instant strike without heating the filaments. However, using high voltage to instant strike a lamp will eventually damage an emissive coating usually applied to the filaments and thus shorten lamp life. Additionally, due to the higher voltage, the instant strike lamps are more costly and less efficient than heated filament lamps.
One of the most common ballasts used in rapid start fluorescent lamps is a two lamp F40T12 rapid start ballast made by MagneTek Universal Mfg., which typically uses about 87 watts of power. In a fixture using such a ballast, the two lamps have a total of four filaments, each of which require about one watt of heating power. In addition, another watt is lost in the ballast filament transformer windings, with filament heating consuming about 6% of the total power consumed by the lamps and ballast. Turning off the power for filament heating after a lamp has struck does not have a noticeable effect on lamp light output, the only adverse effect being a slight reduction in lamp life. However, the cost of energy saved by removing filament heating after striking exceeds the cost incurred due to slightly shortened lamp life.
Many schemes have been proposed over the years for reducing filament heating requirements. For example, U.S. Pat. No. 2,330,312 to Raney shows a relay in series with a lamp which electrically isolates a filament after the lamp strikes. Such series devices have problems in maintaining long term contact reliability, and still consume some power after the lamps are struck. U.S. Pat. No. 4,010,399 to Latassa shows a triac in series with each lamp filament, the triacs being responsive to the difference in filament current before and after the lamp is struck, to reduce power consumption. This circuit is, however, sensitive to changes, with temperature and time, in triac trigger current and lamp filament voltage. Also, these circuits typically experience unit to unit differences. U.S. Pat. No. 4,399,391 to Hammer teaches a separate filament transformer in series with a capacitor and a bilateral voltage sensitive switch such as a sidac. Such a combination of components requires additional housing space and still consumes some power.